Home / CIE AS & A Level Physics : 9.2 Potential difference and power – Exam style question – Paper 1
The total energy supplied to an electric motor is \(E\). Energy \(Q\) is wasted and the remaining energy does useful work.
What is the efficiency of the motor?
Correct Answer: \( \boxed{\mathrm{C}} \)
Efficiency is the ratio of useful energy output to total energy input.
Useful energy \(=E-Q\).
Hence,
\( \eta=\dfrac{E-Q}{E}=1-\dfrac{Q}{E} \).
Therefore, the correct answer is (C).
A copper wire of length \(2\,\mathrm{m}\) has a circular cross-section. There is a constant current in the wire when it is connected to a mobile phone in order to charge the battery.
The wire has a fault. A \(1\,\mathrm{m}\) length of the wire has half the diameter of the other \(1\,\mathrm{m}\) length.
The power dissipated in the thicker length of wire is \(P\).
What is the power dissipated in the thinner length of wire?
Correct Answer: \( \boxed{\mathrm{D}} \)
Since both sections carry the same current, \(P=I^2R\).
For wires of equal length and material, \(R\propto\dfrac{1}{A}\).
Halving the diameter reduces the cross-sectional area by a factor of \(4\), so the resistance becomes four times larger.
Hence, the thinner section dissipates four times the power of the thicker section.
Therefore, the correct answer is (D).
Which circuit symbol does not represent an electric component that is designed to emit sound waves?
Correct Answer: \( \boxed{\mathrm{B}} \)
Symbols A, C and D represent sound-emitting components such as loudspeakers, buzzers or sounders.
Symbol B represents a resistor, which is not designed to emit sound waves.
Therefore, the correct answer is (B).